Electroplating electrolyte and method



United States Patent Ofilice 3,074,859 Patented Jan. 22, 1963 3,074,859 ELECTROPLATIN G ELECTROLYTE AND METHOD Alfons A. Latawiec, Garwood, and George H. Lockwood, Bloomfield, N.J., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporatron of Pennsylvania No Drawing. Filed Aug. 4, 196%, Ser. No. 47,382 5 Claims. ((11. 204-43 This invention relates to an electrolyte and method for electrodeposition and, more particularly, to an electroplating electrolyte and method for electrodepositing black chromium-vanadium coatings.

- It is disclosed in US. Patent No. 2,824,829, dated February 25, 1958, to electrodeposit black chromium-vanadium coatings onto metallic members. It is also disclosed in US. Patent No. 2,885,587, dated May 5, 1959, to provide lead-in members for fluorescent lamps with a black chromium-vanadium coating, in order to improve the lamp performance characteristics with respect to inhibiting envelope end discolorations. The methods by which these coatings are applied, as described in the foregoing patents, produce very satisfactory results, but are relatively slow. It is desirable to apply such black chromium-vanadium coatings to fluorescent lamp lead-in wires, for example, at high-speed production rates, in order to decrease the cost of the resulting product. It is also desirableto useas little of the coating as possible, while still achieving the beneficial effects thereof, in order to increase production rates and decrease production costs.

It is the general object of this invention to provide an improved electroplating electrolytic solution for depositing black chromium-vanadium plating onto a metallic member, which improved plating solution is adapted to high-speed plating.

It is another object to provide an improved electroplating electrolytic solution which can be used to deposit a relatively thin but effective black chromium-vanadium plating onto a metallic member.

It is a further object to provide a process for electrodepositing, under high-speed production conditions, a black chromium-vanadium plating onto a metallic member.

The aforesaid objects of the invention, and other objects which will become apparent as the description proceeds,

are achieved by providing an aqueous electroplating solu tion wherein the essential ingredients which comprise the improved solution are set forth. It has been found that the plating solution enables the black chromium-vanadium coating to be deposited in a very thin and efiicient manner under high-speed production conditions. There has also been provided a method for electrodepositing such acoating, using the foregoing electrolytic plating solution.

While the present electroplating method and electrolyte solution are adapted to plate any metallic member, the invention has particular utility with respect to electroplating nickel, iron, or nickel-iron wire having a diameter of mils, and hence it will be so described.

Previous to plating in accordance with the present invention, the wire is anodically cleaned, first by passing it through an alkaline solution and thereafter passing it through an acid solution. As an example, the Wire is first made the anode in an alkaline cleaner, such as five ounces of sodium hydroxide per gallon of water maintained at a temperature of about 140 F. The wire is passed through this alkaline cleaner in seconds, maintaining a current density of 150 amperes per square foot of wire surface. The wire is then passed through an acid cleaner, such as a 50% aqueous solution of sulphuric acid at room temperature. The wire is passed through this acid cleaner in 30 seconds, maintaining a current density of 200 amperes per square foot of wire surface. Thereafter, the wire is 2 Lhoroughly washed by passing it through a water rinsing ath.

In accordance with the present invention, the cleaned wire is passed through at least one black chromium-vanadium electrolytic plating bath. Since the present invention is particularly adapted for high-speed production plating, the specific values as given herein are for such type of production. Preferably, two tandem plating tanks are used, each having a total length of 44 inches. The wire to be plated is made the cathode by contact spools at the exit side of each tank and sixteen wire strands are simultanously plated. The anode preferably is formed of lead and preferably has a generally convex configuration with respect to the strands of wire being plated in order to deposit a uniform coating on all strands. As a specific example, each of the sixteen strands of wire are separated from the closest wire strand by approximately threeeighths inch. The center wire strands are spaced from the anode by about one-half inch and the end wire strands are each spaced from the anode by about three inches. Wire is continuously passed through the plating tank at a rate of 7.5 feet per minute and sixteen strands of wire completely traverse both tanks in less than one minute. After plating, the wire is thoroughly water rinsed, dried and spooled for use.

The present electrolytic plating solution consists essentially of the following in the stated proportions: from 325 grams to 400 grams of chromic acid (Cr O per liter of solution; from 3 to 6.5 grams per liter 'of solution of vanadium in the form of water-soluble, generally neutral vanadium compound; water-soluble carboxylic acid in such amount as to provide a hydrogen ion concentration equal to that of glacial acetic acid in amount, of from 5 to 10 cc. per liter of solution; and sufficient sulfate-I radical-containing, Water-soluble, non-alkaline ionizable compound to produ-ce from 0.11 to 0.47 gram of sulfate radical per liter of solution. As an example, the vanadium compound is ammonium metavanadate used in concentration of 7 to 15 grams per liter of electrolyte, the preferred water-soluble carboxylic acid is glacial acetic acid, and the sulphate radical is preferably supplied by adding sulphuric acid. As a specific preferredexample,

the electrolytic plating solution consists essentially of about 375 grams of chromic acid perliter-of solution, about 12.5 grams of ammonium metavanadate per liter of solution, about 8 cc. of glacial acetic acidper liter of solution and about 0.25 gram of sulphate radical per liter of solution which is supplied by adding C.P. concentrated sulphuric acid in amount to obtain this concentration of sulphate radical. During plating, the temperature of theelectrolytic solution is maintained at from 77 F. to 90 F.

by means of a conventional cooling system, such as'cooling pipes located in the bottom of the electroplating tank. The current density is maintained at from about 310 to about 340 amperes for the 16 strands being plated and this is equivalent to fromabout- 400 to about 445 amperes per square foot of wire "surface area. Thapotential applied between anode and cathode is approxi-- mately ten volts.

It has been found that the specified relatively high concentration of chromic acid is necessary if adequate plating materialis to be deposited in .a relatively short plating period. With respect to the ammonium metavanadate and glacial acetic acid, if they are greater or less than the tolerable indicated concentrations, the quantity of the black chromium-vanadium plating deposit is diminished. The presence of the sulphate radical has been found to be quite critical. If the sulphate radical is present in less than 0.11 gram per liter of plating solution, the amount of material which is deposited will be considerably diminished. If the sulphate radical concentration is greater than 0.47 gram per liter of plating solution, more coating material will be deposited, but it will assume an increasingly grey appearance. If the concentration of the sulphate radical is greatly increased, the deposit d coating has the appearance of metallic chromium. For highspeed electroplating, it is necessary to maintain the sulphate radical within the foregoing specified concentration limits if a good plating is to be obtained. From a production standpoint, it is desirable for the operator to maintain the specific gravity of the plating solution at from 1.246 to 1.270, measured at a temperature of 75 F. This will provide an excellent control to indicate when it is necessary to add additional chromic acid and ammonium metavanadate.

The foregoing electrolytic solution is subject to some modification. As an example, other water-soluble carboxylic acids can be substituted for the preferred glacial acetic acid, maintaining the hydrogen ion concentration equivalent to that obtained with the glacial acetic acid. As an example, formic acid can be substituted for the glacial acetic acid, decreasing the amount required in proportion to its dissociation constant. Also, while it is preferred to add the sulphate radical as sulphuric acid, other water-soluble, non-alkaline, ionizable compounds can be used in place of the sulphuric acid, examples being sodium or potassium sulphates. In addition, the sulphate radical can be added in the form of vanadyl sulphate, decreasing the ammonium metavanadate a corresponding amount in order that the vanadium concentration is maintained within the foregoing prescribed limits.

Coatings produced in accordance with the present method have a minimum thickness of 0.009 mil and a maximum thickness of 0.016 mil. These thicknesses are considerably less than those of previously reported similar coatings. These very thin coating thicknesses have been found to be quite satisfactory, however, and the coatings obtained always have a black appearance and are extremely adherent to the wire substrate. Apparently the most significant ingredient of the present electrolytic plating solution is the presence of the sulphate radical within the prescribed limits. This, when taken in conjunction with the increased chromic acid concentration, permits the plating to be deposited in a very rapid fashion with good results. In actual performance tests wherein the plated wire was used as lead-in conductors for fluorescent lamps, the plating has been found to be extremely adherent and to function very well to inhibit the first appearance of dark deposits at the ends of the lamp envelopes. The plating in the vicinity of the lamp electrodes is subjected to considerable heat and intense ultraviolet radiations. Neither the heat nor the ultraviolet radiations appear to affect the stability or adherence of the very thin plating.

It will be recognized that the objects of the invention have been achieved by providing an improved electroplating electrolytic solution for depositing black chromium-vanadium plating onto a metallic member. This electrolytic solution is adapted to high-speed production and will deposit a relatively thin but very effective black chromium-vanadium plating. There has also been provided a process for electrodepositing, under high-speed production conditions, a black chromium-vanadium plating onto a metallic member.

While best examples of the invention have been described hereinbefore, it is to be particularly understood that the invention is not limited thereto or thereby.

We claim:

1. An aqueous electroplating electrolytic solution con- 4. sisting essentially of the following in the stated proportions: from 325 to 400 grams of chromic acid per liter of solution; from 3 to 6.5 grams per liter of solution of vanadium in the form of water-soluble, substantially neutral vanadium compound; water-soluble carboxylic acid in such amount as to provide a hydrogen ion concentration equal to that of glacial acetic acid in amount of from 5 to 10 cc. per liter of solution; and sufiicient sulphate radical-containing, water-soluble, non-alkaline, ionizable compound to produce from 0.11 to 0.47 gram of sulphate radical per liter of solution.

2. An aqueous electroplating electrolytic solution consisting essentially of the following in the stated proportions, from 325 to 400 grams of chromic acid per liter of solution, from 7 to 15 grams of ammonium metavanadate per liter of solution, from 5 to 10 cc. of glacial acetic acid per liter of solution, and sufficient sulphuric acid to provide from 0.11 to 0.47 gram of sulphate radical per liter of solution.

3. An aqueous electroplating electrolytic solution consisting essentially of the following in the stated propor! tions, about 375 grams of chromic acid per liter of solution, about 12.5 grams of ammonium metavanadate per liter of solution, about 8 cc. of glacial acetic acid per liter of solution, and sufiicient sulphuric acid to provide about 0.25 gram of sulphate radical per liter of solution.

4. The process of electrodepositing an adherent stable black finish on a clean metallic member, which process comprises: making the member tobe coated the cathode in an aqueous electroplating electrolyte consisting essentially of the following in the stated proportions: from 325 to 400 grams of chromic acid per liter of electrolyte, from 3 to 6.5 grams per liter of electrolyte of vanadium in the form of water-soluble substantially neutral vanadium compound, water-soluble carboxylic acid in such amount as to provide a hydrogen ion concentration equal to that of glacial acetic acid in amount of from 3 to 10 cc. per liter of electrolyte, and suflicient sulphate-radical-containing water-soluble non-alkaline ionizable compound to produce from 0.11 to 0.47 gram of sulphate radical per liter of electrolyte; continuously passing the member to be coated through the electrolyte; and simultaneously passing a plating current through the member at a current density of from 400 to 450 amps. per square foot of member, with the electrolyte maintained at a temperature of from 77 F. to F.

5. The process of electrodepositing an adherent stable black finish on a clean metallic wire, which process comprises: making the wire to be coated the cathode in an aqueous electroplating electrolyte consisting essentially of the following in the stated proportions: from 325 to 400 grams of chromic acid per liter of electrolyte, from 7 to 15 grams of ammonium metavanadate per liter of elec.

References Cited in the file of this patent UNITED STATES PATENTS Westbrook Mar. 10, 1931 Quaely Feb. 25, 1958 

1. AN AQUEOUS ELECTROPLATING ELECTROLYTIC SOLUTION CONSISTING ESSENTIALLY OF THE FOLLOWING IN THE STATED PROPORTIONS: FROM 325 TO 400 GRAMS OF CHROMIC ACID PER LITER OF SOLUTION; FROM 3 TO 6.5 GRAMS PER LITER OF SOLUTION OF VANADIUM IN THE FORM OF WATER-SOLUBLE, SUBSTANTIALLY NEUTRAL VANADIUM COMPOUND; WATER-SOLUBLE CARBOXYLIC ACID IN SUCH AMOUNT AS TO PROVIDE A HYDROGEN ION CONCENTRATION EQUAL TO THAT OF GLACIAL ACETIC ACID IN AMOUNT OF FROM 5 TO 10 C.. PER LITER OF SOLUTION; AND SUFFICIENT SULPHATERADIAL-CONTAINING, WATER-SOLUBLE, NON-ALKKALINE, IONIZABLE COMPOUND TO PRODUCE FROM 0.11 TO 0.47 GRAM OF SULPHATE RADICAL PER LITER OF SOLUTION. 